1. Field of the Invention
The present invention relates to a heartbeat detecting apparatus that detects a heartbeat of a person seated on a seat based on output signals of piezoelectric sensors provided at the seat.
2. Description of the Invention
There is a heartbeat detecting apparatus for detecting a heartbeat of a person seated on a seat of a vehicle, such as an automobile. Heartbeat detecting apparatuses are categorized into an apparatus of constrained type, in which a sensor for detecting the heartbeat of the person is attached at an arm of the person, or the like, and an apparatus of unconstrained type, in which a sensor for detecting the heartbeat of the person is not attached at the person. The heartbeat detecting apparatus of constrained type surely detects the heartbeat when the sensor is attached at a predetermined position of the person. However, because a portion of the person is constrained, the person is prevented from moving freely and a drawback may occur that a subject feels that his/her heartbeat is being detected. On the other hand, the heartbeat detecting apparatus of unconstrained type is preferable in respect that the person is allowed to move freely and the subject does not feel that his/her heartbeat is being detected.
In a heartbeat detecting apparatus of unconstrained type according to patent publication 1, a piezoelectric sensor is provided at a seat of a vehicle. The piezoelectric sensor detects a pulse (i.e. a heartbeat) of a blood flow of a person seated on the seat. More specifically, a frequency analysis is executed on an output signal of the piezoelectric sensor and a power spectrum is calculated. The heartbeat of the person seated on the seat is detected by extracting a signal component corresponding to a heartbeat frequency, from the power spectrum.    Patent publication 1: JP2004-345617A
Such drawback may occur in the heartbeat detecting apparatus of the unconstrained type that the piezoelectric sensor outputs a signal based on a factor other than the heartbeat because the heartbeat detecting apparatus of unconstrained type does not constrain the person. For example, signal components resulted from, such as a movement of the person on the seat (a body movement) and a placement of an object on the seat, may be included in a frequency range of the heartbeat. Therefore, the heartbeat detecting apparatus according to patent publication 1 determines all the signal components among the output signals of the piezoelectric sensor corresponding to the heartbeat frequency to be the heartbeats. However, as described above, signals resulted from, such as the movement of the person and the placement of the object may be included in the signal components of the heartbeat. In particular, signals resulted from the placement of the object and a vibration of a vehicle in an idling state may be determined as the heartbeat signals. In other words, the heartbeat detecting apparatus according to patent publication 1 has a drawback that the heartbeat signal of the person is not precisely obtained because the heartbeat detecting apparatus determines the heartbeat signal without determining whether the occupant is seated on the seat or not.
An objective of the present invention is to provide a heartbeat detecting apparatus that detects whether an occupant is seated on a seat or not, based on an output signal of a piezoelectric sensor. A further objective of the present invention is to provide the heartbeat detecting apparatus that precisely obtains a signal component including a waveform of a heartbeat of the person, from the output signal of the piezoelectric sensor.